In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting material (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting material can be obtained.
Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, the display device also has advantages in that it can be formed to be thin and lightweight, and has high response speed.
In a light-emitting element (e.g., an organic EL element) whose EL layer contains an organic material as a light-emitting material and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic material having a light-emitting property is brought into an excited state to provide light emission.
Note that an excited state formed by an organic material can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The formation ratio of S* to T* in the light-emitting element is 1:3. In other words, a light-emitting element including a compound emitting phosphorescence (phosphorescent compound) has higher light emission efficiency than a light-emitting element including a compound emitting fluorescence (fluorescent compound). Therefore, light-emitting elements containing phosphorescent materials capable of converting energy of the triplet excited state into light emission have been actively developed in recent years (e.g., see Patent Document 1).
Energy for exciting an organic material depends on an energy difference between the LUMO level and the HOMO level of the organic material. The energy difference approximately corresponds to singlet excitation energy. In a light-emitting element containing a phosphorescent organic material, triplet excitation energy is converted into light emission energy. Thus, when the energy difference between the singlet excited state and the triplet excited state of an organic material is large, the energy needed for exciting the organic material is higher than the light emission energy by the amount corresponding to the energy difference. The difference between the energy for exciting the organic material and the light emission energy affects element characteristics of a light-emitting element: the driving voltage of the light-emitting element increases. Research and development are being conducted on techniques for reducing the driving voltage (see Patent Document 2).
Among light-emitting elements including phosphorescent materials, a light-emitting element that emits blue light in particular has not yet been put into practical use because it is difficult to develop a stable organic material having a high triplet excited energy level. This has motivated the research effort to develop highly reliable light-emitting elements that exhibit phosphorescence with high emission efficiency.